A thin film transistor has been well known as a thin film lamellar device for liquid crystal display apparatuses (hereinafter, just referred to as LCDs). By equipping an LCD of an active matrix type with the thin film transistor as a switching element, it is possible to realize high-speed moving picture and subtle display, which are properties required in the LCD. Note that the LCD of the active matrix type is an LCD in which a switching element is provided for each pixel so as to control electric charges applied on pixel electrodes.
The LCD using the thin film transistor is so arranged as to have a glass substrate or a quartz substrate in order to be tolerant against thermal process, chemicals and the like in forming the thin film transistor.
Especially, in a portable information terminal, which is in increasing demand recently, the LCD using the thin film transistor is provided. The use of a reflective LCD as the LCD in the portable information terminal eliminates the need of a back light and reduces power consumption in the portable information terminal.
Note that the reflective LCD is an LCD provided with (i) a first insulating substrate including scanning lines and signal lines crossing each other in matrix, a thin film transistor and a reflective electrode, and (ii) a second transparent insulating substrate including a color filter layer, a black matrix, and a counter electrode, the first and second insulating substrates being pasted together face-to-face (panel alignment), and (iii) a twist nematic (TN) liquid crystal filled in-between the first and second insulating substrates.
The LCD using the thin film transistor is applied in various fields. The various application of the LCD verifies required characteristics of the same. Especially, it is important to improve the LCD to have lighter weight, better shock resistance, or lower cost, because the demand of the LCD for use in the portable information terminal is increasing.
However, in the conventional LCD in which the thin film transistor is provided on the glass substrate or quartz substrate, there is a limit in thickness of the substrate. The limit prohibits the conventional LCD from having a substrate thinner than the limit. Moreover, if a thinner glass substrate or the quartz substrate is used for reducing the weight of the LCD, the substrate becomes fragile and less resistant against shock. Further, cost of the glass substrate or quartz substrate gives a limit in cost reduction of the LCD.
In short, it is difficult to improve the thin film transistor using the conventional glass substrate or the like for lighter weight, better shock resistance, and lower cost, which are required characteristics for the LCD.
For realizing lighter weight, better shock resistance, an lower cost in the LCD, there has been an attempt to use a plastic substrate in a thin film transistor.
Incidentally, the conventional reflective LCD has the following problems.
Namely, in the conventional reflective LCD, it is necessary to accurately paste the first insulating substrate having the thin film transistor and the second insulating substrate having the color filter layers together in order to prevent light leakage and color blurring due to incorrect positioning in bonding.
As solution for this, Japanese Publications of Unexamined Patent Application, Tokukai No. 2000-162625 (published on Jun. 16, 2000), and Tokukai No. 2000-187209 (published on Jul. 4, 2000) disclose a color filter layer-on-array-structure in which the color filter layers and the thin film transistors are formed on an insulating substrate.
According to methods described in the publications, it is possible to form the color filter layers on the substrate having the thin film transistors. Therefore, according to the methods, it is possible to manufacture the LCD without a fear of the incorrect positioning in pasting the first insulating substrate having the thin film transistor with the second insulating substrate including the color filter layers.
However, if the plastic substrate is used as the insulating substrates, the plastic substrate is expanded and contracted by heat and humidity in a step of forming the color filter layers, or a step of forming the pixel electrode.
In short, in the prior arts disclosed in the publications, if the plastic substrate is used as the insulating substrates, it is difficult to attain accurate matching in forming the color filter layers on the reflective electrode on the substrate. This leads to incorrect positioning, which causes light leakage and color blurring, thereby deteriorating display image in quality.
In other words, it is difficult to improve the conventional reflective LCD in cost or the like by using the plastic substrate while preventing deterioration of the display image in quality.